In mining, tailings or tails are the materials left over after the process of separating the valuable fraction from the uneconomic fraction (gangue) of an ore. Tailings are different from overburden, which is the waste rock or other material that overlies an ore or mineral body and is displaced during mining without being processed.

The extraction of minerals from ore can be done two ways: placer mining, which uses water and gravity to concentrate the valuable minerals, or hard rock mining, which pulverizes the rock containing the ore and then relies on chemical reactions to concentrate the sought-after material. In the latter, the extraction of minerals from ore requires comminution, i.e., grinding the ore into fine particles to facilitate extraction of the target element(s). Because of this comminution, tailings consist of a slurry of fine particles, ranging from the size of a grain of sand to a few micrometres. Mine tailings are usually produced from the mill in slurry form, which is a mixture of fine mineral particles and water.

Tailings can be dangerous sources of toxic chemicals such as heavy metals, sulfides and radioactive content. These chemicals are especially dangerous when stored in water in ponds behind tailings dams. These ponds are also vulnerable to major breaches or leaks from the dams, causing environmental disasters. Because of these and other environmental concerns such as groundwater leakage, toxic emissions and bird death, tailing piles and ponds are often under regulatory scrutiny. There are a wide range of methods for recovering economic value, containing or otherwise mitigating the impacts of tailings. However, internationally, these practices are poor, sometimes violating human rights. To mitigate risks of harm, the first UN-level standard for tailing management was established 2020.